The MC21 has quite a different linkage to the MC18. Check here http://www.nsr250.net/forums/viewtopic.php?t=8292&highlight=

I tried to fit the MC21 Shock to my 18 and it would not go even tho they are not that different in length.

I had it cut down to length and rebuilt by a suspension expert and it seems really good on the road. Not too expensive either from memory about $140 NZ or 70ish pounds